Typically, thick film heaters are composed of four main layers; a metallic substrate, an insulating layer, a resistive layer coated on the insulating layer and an overglaze layer. For some specific applications, it is very important to heat the plate in a very short time with high temperature uniformity. To meet these requirements, the track pattern needs to be designed with special care.
Achieving high temperature uniformity and short heating up time with limited power consumption in a heater is related with the construction materials properties such as thermal conductivity, thermal expansion coefficient, specific heat and density. So, heater plate constructors try to combine different construction materials in order to diminish their interrelated obstacles.
In many heating plate designs, an additional layer has to be applied to eliminate. various disadvantages of using substrates. In the U.S. Pat. No. 6,222,166, heating plate uses aluminum substrate due to its exceptional thermal conductivity and uniform heat distribution characteristics. Since the substrate has a very high thermal expansion coefficient, an insulator layer is applied over the substrate. However, it is important to note that proposed additional layers result in high heat capacity due to increased mass and volume which is not favorable regarding power consumption and required time to reach desired temperatures. The increased mass and volume also make the heater plate not appropriate for some low volume applications.
Moreover, an ideal heater plate has to have compact track pattern of resistive layer in order to reduce the volume and the power consumption. However, tight turns of the resistive track pattern causes non-homogenous distribution of current density through the pattern called “current crowding” phenomenon. Non-homogenous distribution of current density can lead to localized overheating and formation of thermal hot spots. In some extreme cases it is leading to a vicious circle like thermal runaway. The rising temperature can also leads to localized thermal expansion on the material. As a result of localized thermal expansion, a big stress occurs at the joint parts and some cracks emerge or split apart at the joint which also causes short circuits.